1) Field of the Invention
The present invention relates to a circuit unit incorporated in an electronic instrument having a key-pad arrangement, such as a portable telephone.
2) Description of the Related Art
In general, such a circuit unit comprises a printed circuit board, a flexible switch circuit sheet attached to the printed circuit board and having a plurality of switch pads arranged thereon, and a key pad board placed on the flexible switch circuit sheet and having a plurality of key pads correspondingly arranged thereon with respect to the arrangement of the switch pads. The flexible switch circuit sheet has at least one circuit pattern formed therein to construct a switch circuit together with the switch pads. The circuit pattern includes a plurality of line sections extended between the switch pads, and a plurality of terminals formed on the flexible switch circuit sheet for establishing an electrical connection between the flexible switch circuit sheet and the printed circuit board. Namely, the terminals of the circuit pattern are soldered to terminal pads formed on the printed circuit board, so that a switch on the flexible switch circuit sheet is electrically connected to a control circuit on the printed circuit board. Thus, when the switch pad concerned is depressed by the corresponding key pad and turned ON, the signal can be received by the control circuit of the printed circuit board.
Conventionally, the terminals of the circuit pattern are provided on and gathered at, a localized zone of the flexible switch circuit sheet, and thus the total length of the line sections included in the circuit pattern becomes relatively long, because the line sections must be extended to the terminals provided at the localized zone. Of course, the total length of line sections should be reduced as much as possible, so that the switch circuit can be more effectively protected from noise. Also, the longer the total length of the line sections of the circuit pattern, the larger the size of the flexible switch circuit sheet.
Furthermore, a flexibility of the flexible switch circuit sheet is not uniform because the line sections included therein are concentrated toward the gathered terminals. Accordingly, when the switch pad concerned is depressed by the corresponding key pad, a concentration of stress occurs in the circuit unit so that it is susceptible to local damage.